(Greek, Keltoi), tribes closely related in language (seeCELTIC LANGUAGES) and material culture who initially inhabited, during the first half of the first millennium B.C., the basins of the Rhine, Seine, and Loire and the Upper Danube and who later settled what is now France, Belgium, Switzerland, the southern Federal Republic of Germany, Austria, northern Italy, northern and western Spain, the British Isles (the Celts of Britain were called Britons), Bohemia, and parts of Hungary and Bulgaria. The Romans called them Galli (Gauls), and the name of the principal territory they settled—Gaul—is derived from this usage. The Celts who penetrated Asia Minor during the third century B.C. were called Galatians.

There were two periods in the development of the material culture of the Celts: the Hallstatt culture (900–400 B.C.), followed by the La Tène culture (second half of the first millennium B.C.). After being dispersed, the Celts merged with local tribes—Iberians, Illyrians, Thracians, and others. The development of the Celts of southern France was related to the ancient city-states, and hence they achieved a higher level of culture. Expelled by the Romans from northern Italy during the second century B.C., the Celts established themselves in central and northwestern Bohemia (these were the Boii tribes, from which the area received the name of Bohemia). The most important Celtic tribes were the Helvetii, Belgae, Sequani, Lingones, Aedui, Bituriges, Arverni, Allobroges, Senones, Treviri, and Bellovaci. Farming and stock raising played a large role in the economic life of the Celts. Also highly developed was the production of metal, glass, leather, and ceramics as well as shipbuilding. The Celts probably invented the heavy plow with a colter.

Celtic art was distinguished by geometric, floral, and zoomorphic ornamentation on metal—engraved in its early period and later in relief, combined with inlay and enamel work. Greatly reworked borrowings from Greek and Roman art were characteristic of Celtic metal and stone sculpture (stylized masks and figures of deities, heroes, beasts, birds, and fantastic creatures). The buildings of the Celts were, for the most part, primitive: semi-subterranean dwellings; the simplest, frame out-buildings and workshops; and rectangular sanctuaries. The Celts, primarily those in southern France, built hill forts (oppida) with stone structures surrounded by massive walls of stone blocks. These subsequently became city-fortresses and trade and artisan centers (Bibracte, Gergovia, Alesia, Stradonice).

The principal social units of the Celts were the pagi—territorial districts of individual communities whose members were related by blood. The druids had great influence among the Celts; concentrated in their hands were the religious offices, the highest juridical authority, and education.

Celtic tribes were at various stages in the disintegration of the communal-clan system. Traditions of the clan-tribal organization were especially strong among the Belgae and the Aquitanian tribes in Gaul and among the Celts of the British Isles. In the more developed tribes, responsible officials began to emerge; a tax system appeared along with other attributes of a state organization.

Internecine wars, which weakened the Celts, facilitated the invasion by Germanic tribes from the east and the Romans from the south. During the first century B.C. the Germanic tribes drove part of the Celts beyond the Rhine. In the period 58–51 B.C., Julius Caesar conquered all of Gaul. During the reign of Augustus the Romans conquered the areas along the Upper Danube, northern Spain, and Galatia, and during the reign of Claudius (the middle of the first century A.D.), a considerable portion of Britain. While part of the Roman Empire, the Celts underwent intensive Romanization. Beginning as early as the first century B.C., ceramic, glass, and bronze ware of the Roman type was widespread among the Celts; subsequently, Gallo-Roman architecture and sculpture emerged (seeFRANCE). The Celtic traditions of ornamentation and metalworking were preserved during the first millennium A.D., particularly in Ireland.